I'm from Australia too, and a month or so ago I overheard a parent calling out to their 3- or 4-year-old son, Banjo. I knew it was getting popular hear but I'd never actually heard it in real life before.
I wouldn't use it either, but I don't know how much I like it. It's OK, but a bit weird. Banjo Paterson is fine, but it's still an instrument. (I'm a bit torn about that, because I do like the name Viola!)
1. Viola and Banjo would be fantastic. In seriousness, though:
Chester
Aspen
Arden
Oliver
Dashiell
August
Otto
Tristan
Elliot
Griffin
Montague
Lorcan
Xander
River (full name Snowy-River Manfrom Lastname) (Although River and Banjo do actually sound good together)
Maisie
Beatrice
Delilah
Edith nn Edie
Juniper
Juno
Kaia
Lola
Posey
Penelope
Tess
Aurora
Camille
Clara
Avalon
2. Banjo Caspian
Banjo Julius
Banjo Torin
3. I don't like it too much, but I wouldn't say it's silly. In my head, it's kind of the Australian answer to Atticus. It sounds sillier than Atticus.